Recently, disk recorders have replaced VTRs to record television broadcasts. The disk recorder records a moving image signal on a disk medium such as a hard disk or DVD, and plays back the signal. The disk recorder can record digital moving image data on one disk medium by encoding the data by variable-length coding such as MPEG-2 and compressing an enormous amount of information.
In compression based on variable-length coding, the data rate (bit rate) of encoded data changes any time in accordance with the complexity and motion of an input image. Generally, in order to prevent buffer overflow and underflow, and meet the limitation of the recording capacity, the coding bit rate is controlled to adjust the encoded data amount close to a predetermined value by feedback control (see, e.g., Japanese Patent Laid-Open No. 2000-197048 (corresponding to U.S. Pat. No. 6,650,787)).
By the difference in coding bit rate control method, variable-length coding processes are classified into two coding methods: CBR (Constant Bit Rate) coding which keeps the generated code amount almost constant, and VBR (Variable Bit Rate) coding which performs optimal code amount distribution in accordance with the complexity and the degree of motion of an image in each frame of a moving image even while adjusting the average value of the coding bit rate to a target value.
In VBR coding, the generated code amount of all moving image data to be coded is estimated, and then actual coding is performed. With this 2-pass arrangement, ideal code amount distribution can be achieved. Conventionally, moving image data to be encoded is temporarily accumulated, and undergoes VBR coding by offline processing by software.
With recent development of hardware which performs VBR coding in real time, the above-mentioned disk recorder generally executes real-time VBR coding, which is advantageous to the image quality. This real-time bit rate control technique is called 1-pass encoding in contrast to 2-pass encoding of distributing a code amount in advance to moving image data to be encoded and encoding the data.
Control of the coding bit rate in VBR coding by 1-pass encoding tries to implement almost ideal code amount distribution, which is achieved by 2-pass encoding, regardless of local characteristics such as the complexity and the degree of motion of each frame of a moving image. For this purpose, coding bit rates in a predetermined period are averaged, and the code amount is gradually controlled to adjust the average value to a target coding bit rate. The gradient of control to make the average bit rate asymptotic to a target value determines the time of convergence to the target coding bit rate.
FIG. 14 is a timing chart showing an example of a change of the coding bit rate when a target bit rate is set and moving image data is encoded by VBR coding. L1401 represents a change of the coding bit rate per unit time. In L1401, the coding bit rate gradually converges to a target bit rate BR in a convergence time T.
As a method of controlling the coding bit rate to a target bit rate in a predetermined convergence time in VBR coding, a convergence curve corresponding to the target bit rate is selected on the basis of the time elapsed after the start of coding and the current average bit rate. While the influence on image quality is reduced, the coding bit rate is made to converge to the target bit rate in a predetermined convergence time (see, e.g., Japanese Patent Laid-Open No. 2000-197048).
When a television broadcast is received and recorded by a disk recorder or the like, the time of convergence to a target bit rate in VBR coding is considered to be ideally around 10 min, in order to eliminate any dependence on local characteristics of a moving image and adjust distribution to an ideal one.
At this time, the average of all bit rates after the start of recording is calculated and controlled to a target coding bit rate, assuring a total recording data amount at a predetermined precision. This control is effective when the free capacity of a medium and the recording time are defined in timed recording of a television broadcast or the like.
In recent years, video cameras using DVDs as recording media are also available. In the video camera, one scene from the start to stop of recording takes generally 1 min or less. If the code amount is controlled to adjust the coding bit rate to a target bit rate for a long time in VBR coding, as described above, recording often stops before the coding bit rate converges to the target rate. It is conceived to control the code amount in VBR coding so that the coding bit rate converges to a target bit rate in a short period of 1 min or less in the video camera or the like.
The video camera has a weak demand for assurance of the recording time, compared to a home disk recorder and the like. Some video cameras do not perform strict adjustment of the recording capacity using the total average bit rate after the start of recording.
The above-described video camera has a function of receiving not only moving image data photographed by a camera but also moving image data externally transmitted to the camera via an external input terminal, and encoding and recording the data.
A video camera which encodes and records moving image data by VBR coding performs bit rate control when moving image data externally input to the video camera is recorded, similarly to a case wherein moving image data from the camera is encoded. When a TV tuner or the like is connected to an external input terminal to record a television broadcast program by the video camera, code amount distribution for the total program is less optimal than in a home disk recorder, which is disadvantageous to the image quality and recording capacity.
The above-mentioned video camera which does not adjust the recording capacity using the total average bit rate performs the same bit rate control even when externally input moving image data is encoded. No actual recording time can be assured for display of the recordable time at the start of recording.
Recently, digital cameras using memory cards as recording media, and video cameras using DVDs as recording media are put into practical use. Further, video cameras which have both a DVD drive and memory card slot and can record moving image data on each recording medium also appear on the market.
These days, the recording capacity of memory cards is abruptly increasing, but is limited in comparison with an optical disk (e.g., DVD) and an HDD. One scene from the start to stop of recording often takes 1 min or less.
The code amount is controlled to adjust the coding bit rate to a target bit rate for a long time in VBR coding in a video camera capable of recording moving image data on both a disk medium of a relatively large recording capacity and a memory card of a small recording capacity. In this case, if moving image data is recorded on a memory card, recording often stops before the coding bit rate converges to a target rate.
When the bit rate is controlled in recording moving image data on a memory card, similar to encoding moving image data on a disk, the recording bit rate greatly varies even in recording on the memory card of a limited recording capacity. The recordable time cannot be accurately predicted, resulting in poor user friendliness.
When the above-mentioned video camera having the VBR coding mode photographs a scene, e.g., a ceremony continuously for a long time, code amount distribution is less optimal as a whole than in a home DVD recorder, which is disadvantageous to the image quality and recording capacity.
If the above-mentioned video camera which does not adjust the recording capacity using the total average bit rate records data continuously for a long time, no actual recording time can be assured for display of the recordable time at the start of recording.
The video camera is controlled to correct exposure at a combination of mainly the F-number and shutter speed within the range of normal brightness. If the shutter speed excessively decreases in a dark scene in which the F-number is set to a full-aperture value, an image blurs. In such a dark scene, the gain of the signal processing circuit must be increased.
The increase in the gain of the signal processing circuit increases noise in an image.
Compared to photographing at normal brightness, coding complexity changes, and code amount distribution in VBR coding becomes less optimal.